The manufacture of grain oriented electrical steels requires critical control of chemistry and processing to achieve the desired magnetic properties in a stable and reproducible manner. The present invention produces excellent magnetic properties in (110)[001] oriented electrical steel having a high volume resistivity.
The specific magnetic property used to evaluate the quality of oriented electrical steel varies with the device manufactured from the steel. However, the highest quality usually implies the lowest core loss at an alternating magnetic field of a specified frequency and amplitude, for example: 60 hertz, 1.5 Teslas. The core loss may be lowered by one or more of the following methods: 1) increasing the volume resistivity through the addition of solute elements (principally silicon); 2) improving the degree of (110)[001] orientation through alloy and process modifications; 3) reducing the final thickness of the steel; 4) improving the purity of the alloy by raw material selection and/or process modifications; 5) improving the magnetic domain structure by one or more process modifications: increasing secondary grain boundary area (reduced secondary grain size and/or increased grain boundary roughness); using a scribing technique; and applying a stress inducing coating.
In recent years, core loss improvements have been made to grain oriented electrical steels which increased the volume resistivity from 47-49 micro-ohm-cm (.mu.-.OMEGA.-cm) to 50-51 micro-ohm-cm. This increase in volume resistivity was obtained by raising the silicon content of the steel from a level of 2.9-3.15 wt % to a level of 3.25-3.5 wt %. This small increase in silicon required intensive development efforts; adjustments were required to alloying elements other than Si; modifications were necessary for process anneals and rolling procedures; and material handling methods had to be improved to accommodate an increased tendency for strip breakage. The practical limit for silicon in a grain oriented steel used in power and power distribution transformers is thought to be 4.5 wt % where the volume resistivity of iron-silicon alloys reaches a level of 63 micro-ohm-cm. Above 4.5 wt % silicon, the procedures associated with the manufacture of grain oriented electrical steel.
A high degree of (110)[001] orientation is achieved in grain oriented electrical steels by processing to obtain selective secondary grain growth which is vigorous enough to consume virtually all grains deviating from the (110)[001] orientation. For secondary grain growth to be both selective and vigorous, a material must have a structure of recrystallized grains with a controlled distribution of orientations, and must have a grain growth inhibitor to restrain primary grain growth in the final anneal until secondary grain growth occurs, typically in the temperature range of 760.degree.-1050.degree. C. (1400.degree.-1922.degree. F.). The production of grain oriented electrical steel relies on the use of precipitates, such as MnS, Mn (S,Se), AlN or combinations of these to act as grain growth inhibitors and may also use minor additions of elements, such as Sb, Cu, Sn and others, which may modify the behavior of the precipitates and/or control the distribution of grain orientations prior to secondary grain growth. The size and spatial distribution of primary grain growth inhibitor precipitates suitable for grain oriented electrical steels has traditionally been provided by a slab or ingot solution treatment immediately prior to hot rolling. The primary grain growth inhibitor precipitates are then formed during the hot rolling operation and/or during subsequent heat treatments.
The traditional processing of oriented electrical steels includes reheating a cooled slab or ingot to temperatures in excess of 1300.degree. C. (2370.degree. F.) prior to hot rolling to a thickness normally less than 3 mm. This high temperature reheating practice allows the MnS, Mn(S,Se) and/or AlN to be dissolved prior to precipitation in a controlled manner during hot rolling and other subsequent processing. However, the high temperature reheating operation is costly, both from the aspect of its destructive effect on equipment and the loss of silicon steel due to the excessive oxidation of the slab or ingot surfaces. Efforts to reduce product loss and protect equipment have included the development of specialized heating equipment. The steel is heated to &gt;1300.degree. C. (2370.degree. F.) in a non-oxidizing atmosphere or the interior of the ingot or slab is heated by induction heating to &gt;1300.degree. C. (2370.degree. F.) while maintaining the surface below 1300.degree. C. (2370.degree. F.). Modified alloy compositions and processes for those alloys have also been developed which allow the use of reheat temperatures below 1300.degree. C. The modified alloys and processes are referred to as "low reheat technologies."
Most of the low reheat technologies include the use of AlN precipitates, either with or without MnS precipitates, as the principle agent for inhibiting primary grain growth in slabs which are hot rolled from a temperature of 1100.degree.-1250.degree. C. A notable exception is the practice taught in U.S. Pat. No. 3,986902 where a conventional grain oriented product is produced using a grain growth inhibitor consisting only of MnS precipitates. U.S. Pat. No. 3,986,902 teaches the use of a reduced product of manganese and sulfur, (% Mn)(% S), combined with a lower total oxygen in order to successfully produce oriented electrical steel from slabs or ingots hot rolled from temperatures of 1250.degree. to 1300.degree. C.
A majority of the grain oriented electrical steel technologies use an initial alloy composition which displays transcritical behavior. The alloy solidifies as ferrite (bcc iron), then, on cooling, becomes a mixture of ferrite and austenite (fcc iron), and on further cooling to &lt;700.degree. C., the austenite decomposes and the alloy becomes essentially ferrite again. Most of the traditional and low reheat technologies use carbon as a temporary alloying agent in Fe--Si alloys containing 2.8 to 3.5% Si such that the alloys exhibit transcritical behavior during hot rolling and/or process anneals and then become fully ferritic when carbon, the temporary alloying agent, is removed in a strip decarburization treatment. The alloys typically reach a peak austenite volume fraction between 0.05 and 0.50 at a temperature between 1100.degree. and 1200.degree. C. Alloys which are fully ferritic prior to the secondary grain growth anneal can be designed and processed such that the secondary growth will occur at temperatures in the range 700.degree.-1100.degree. C.
Alloys which retain transcritical behavior through all manufacturing operations must undergo complete secondary growth at temperatures below 950.degree. C. or formation of austenite (fcc iron) will interfere with the growth of the secondary grains. This temperature range is below that normally associated with secondary grain growth that produces the highest degree of (110)[001] grain orientation. As such, these alloys are believed to have less potential as a displacive technology for the more traditional grain oriented electrical steels. This low secondary growth temperature range also excludes the use of these alloys for the production of a cube texture with a (100)[001] or (100)[hkl] orientation by a secondary growth method; the onset of secondary growth for cubic texture normally occurs above 1000.degree. C. Examples of low reheat technologies which retain transcritical behavior after carbon removal include Fe--Si alloys containing &lt;2% Si (U.S. Pat. No. 4,596,614) or Fe--Si--Mn alloys containing (Si-0.5Mn) &lt;2% (U.S. Pat. No. 5,250,123).
A feature of the low reheat technologies using AlN precipitates as a grain growth inhibitor is the stated or inferred use of a nitriding treatment prior to secondary grain growth. Several technologies actually specify nitrogen levels that must be reached in the steel prior to secondary growth. All of these technologies teach the use of an atmosphere containing nitrogen or a separator coating which includes a nitrogen bearing compound in the secondary growth anneal during heating and secondary growth.
There are several low reheat technology patents which disclose a continuous strip nitriding treatment which may be used during or after decarburization to provide excellent magnetic properties in alloys using AlN and (Al-Si)N precipitates as the grain growth inhibitor. U.S. Pat. No. 4,979,996 had an electrical steel composition containing 0.025-0.075% C., 2.5-4.5% Si, 0.012% max S, 0.01-0.06% Al, 0.01% max N, 0.08-0.45% Mn, 0.015-0.045% P and balance essentially Fe. This patent disclosed the use of a continuous furnace to nitride the strip after the decarbufizing anneal. For nitriding, the strip was held in the temperature range of 800.degree.-850.degree. C., in an atmosphere containing NH.sub.3 and hydrogen for a time of at least 10 seconds and preferably less than 60 seconds. After the strip nitriding process was completed, at least 180 ppm nitrogen was present as averaged through the thickness of the steel. Long times were previously required for nitriding in order to diffuse the nitrogen between the laps of the tightly wound coils. Attempts were also made to nitride in loose coils but these were found to have uneven temperature distributions which caused uneven nitriding conditions.
In traditional grain oriented electrical steels, Mn is combined with S or S+Se to form MnS or Mn(S,Se) precipitates which function as all of, or a significant portion of, the grain growth inhibitor. Manganese is held to levels below 0.15% so that the product of (% Mn)(% S) or (% Mn)(% S+a % Se), where a is an empirically determined constant, is sufficiently low that the inhibitor precipitates may be dissolved entirely in the slabs or ingots prior to hot rolling. Most low reheat technologies rely completely or substantially on AlN precipitates as the grain growth inhibitor. Manganese is controlled to levels below 0.45% and typically less than 0.15%. Other additions may be made which modify the behavior of these precipitates and these include, by way of example, copper, antimony, arsenic, bismuth, tin, nickel and others.
An example of a low reheat technology which uses high manganese is U.S. Pat. No. 5,250,123. This patent discloses the use of a balance of Mn and Si such that (% Si)-0.5(% Mn)&lt;2.0, which causes the claimed alloys to be transcritical without the use of carbon as a temporary alloying element. The steels of this patent had 1.5-3% silicon, 1-3% manganese, 0.002% maximum total for carbon and nitrogen, and 0.003-0.015% soluble aluminum in a grain oriented electrical steel. The soluble aluminum had to be maintained below 0.015% to avoid excessive inhibitors which were poorly dispersed. Silicon above 3% was stated to cause unstable secondary recrystallization and poor workability. The sum of carbon plus nitrogen above 0.002% after a final purification anneal was stated to form carbides and nitrides which obstructed domain wall movement and increased core loss. Manganese above 3% was stated to cause unstable secondary recrystallization and poor workability.
Grain oriented silicon steel has been balanced using compositions which restrict the levels of Si, C, Mn and Al in order to provide a material which is transcritical and may be processed with low slab reheat technology. A product has not been developed which allows high levels of Mn and Si in a transcritical material which has stable secondary grain growth, good workability and high volume resistivity.